A recent mention in another radio forum lead me to this article from Toronto, Canada:
http://www.thestar.com/news/gta/torontog20summit/article/821623--g20-security-to-jam-cellphone-signals?bn=1
Although the article is specifically about the use of mobile cell phone jamming "bubbles" at the Canadian G20 events, the same technology surely has been in use at US based events.
There has been much discussion over the years about the WHCA mobile communications vans around the Presidential motorcades and how they have jamming capabilities. There has not been much said in the media about the use of cell phone jammers by the US Secret Service or WHCA and certainly no mention of getting exemptions from federal law forbidding such jamming.
I have not yet been able to confirm or deny the use of cell phone jamming at POTUS events and such. There were rumors of such jamming at the last set of political conventions in Denver and St. Paul, but I had no personal confirmation of any intentional interference of cell systems. There were areas inside the secured buildings that were considered "RF Sensitive" and cell phones were not allowed, however...